Effects of microwave and conventional blank roasting on oxidation behavior, microstructure and surface morphology of vanadium slag with high chromium content

The effects of microwave blank roasting (MBR) and conventional blank roasting (CBR) on oxidation behavior, microstructure and surface morphology of vanadium slag with high chromium content (VSHCC) containing 12.54 wt% Cr2O3 were investigated. The oxidation behavior, microstructure and surface morphology of samples before and after MBR and CBR were characterized using X-ray diffraction (XRD), fourier transform infrared spectroscopy (FT-IR), simultaneous thermal gravimetric and differential scanning calorimetry (TG-DSC) and scanning electron microscope (SEM), respectively. The MBR experiments were performed in a microwave roasting instrument at temperature ranging from 400 degrees C to 900 degrees C and the CBR experiments were performed in muffle furnace at temperature ranging from 600 degrees C to 900 degrees C. The results show that the normal spinel begins to be oxidized to inverse spinel at a lower temperature and to decompose at 600 degrees C. The completely decomposition temperatures of the normal spinel using MBR and CBR are 700 degrees C and 800 degrees C, respectively. The formation temperatures of Fe2TiO5 using MBR and CBR are 600 degrees C and 900 degrees C, respectively, and the content of Fe2TiO5 and (Fe-0.6, Cr-0.4)(2)O-3 gradually increase as MBR temperature (from 600 degrees C to 900 degrees C) increases. After roasted at a high temperature, the majority of Cr3+ ions in the spinel phase are conjugated to iron ions to form hematite-type (Fe-0.6, Cr-0.4)(2)O-3 solid solutions and the minority of Cr3+ ions in the spinel phase are incorporated into VO2 to form the Cr0.07V1.93O4 or CrVO4. The completely decomposition temperature of the olivine revealed by FT-IR analysis is higher than that concluded from the XRD analysis. The reason is that the minimum detection level of FT-IR analysis is more sensitive than the XRD analysis. Compared with the CBR, the MBR can accelerate the oxidative decomposition of the spinel and olivine phase in the raw slag and also can lower the formation temperature of Fe2TiO5. Besides, it also can reduce the granularity of roasted particles and make the surface become more porous especially at a high roasting temperature. The reason for this phenomenon can be due to the different of heat generation and heat transferring mechanisms in MBR and CBR. Based on the results, a new approach has been proposed to extract vanadium from VSHCC. At last, kinetic analysis indicates that the oxidative decomposition of vanadium chromium spinel phase is controlled by the rate of 0.43 reaction order and the corresponding total apparent activation energy and the frequency factor are 112.57 kJ mol(-1) and 1.86 x 104 min(-1), respectively. (C) 2016 Elsevier B.V. All rights reserved.